The invention relates to a process for the crossflow-microfiltration of wine from a storage container through a membrane filter with membranes having a nominal pore diameter of 0.1-5/.mu.m, in which the mean velocity in the membrane filter is 0.5-6 m/second, and an apparatus for implementation of the process.
All wines are cloudy at the beginning of their development. As they mature, they clear spontaneously, although clarification is slow and incomplete. Currently, clarification is predominantly mechanical.
Substances which cause cloudiness in young wine contain as their chief components yeast, protein compounds, pectins, glucans, tannins, polyphenols and sometimes dyes. The percentage composition fluctuates from one wine to the next, and it also fluctuates greatly from one vintage to the next. For his work, the winemaker has at his disposal a number of permitted refining agents as well as filtration techniques and apparatus that have been accepted for decades.
Despite numerous advances in this area, the current state of technology is still far from the solution of all problems.
The processes in general use at present are actually still emergency solutions, because the most efficient of all the conceivable procedures, viz. "sieving" (filtration) through a "sieve" (filter) with the right "mesh size" (separation threshold), cannot be utilized without special measures because of the extreme fineness of the particles causing turbidity.
The compromises that must be made for the conventional processes give rise to a number of individual problems, at least the following ones of which are serious and therefore in urgent need of solution:
Major expenditures of time, work and finances due to several process stages;
High energy requirement, noise, inadequate separation when using a separator (which can therefore be utilized as a preliminary step only);
Expenditure of time, product loss, expenditures for transportation, storage and compensation due to the use of diatomite filters, which is also increasingly becoming a problem of resources and costs;
Expenditure of time, product loss, expenditures for transportation, storage and compensation for the filters used in fine clarifiction and sterilization.
The crossflow-microfiltration process permits the use of the most efficient method, viz. separation by sieving of the fine particulate substances causing turbidity, which to date has appeared impossible.
The particles causing turbidity occur in various sizes, ranging from the coarse particles of several .mu.m which settle rapidly, largely comprising yeasts of 4-20 .mu.m, to particles of approximately 0.2-1.0 .mu.m. Bacteria are included among the latter. The required threshold of separation is determined by the smallest particles that are to be separated.
As mentioned above, static filtration with the "appropriate sieve," i.e., a membrane filter, is not feasible in this range because filtration would very rapidly come to a standstill, especially in view of the relatively high concentrations of suspended particles. Due to the fineness of the particles and the pressure required for static filtration procedures, there would be rapid formation of a dense and firm filter cake which, even in a thin layer, would be absolutely impermeable to the material to be filtered.
However, if the formation of a filter cake can be prevented or at least greatly limited, e.g., by establishment of constant flow over the filter, then the desired filtration method can again be used. This is known from ultrafiltration in which separation thresholds are considerably lower than those in microfiltration.
By establishment of flow over the membrane filter, the liquid to be filtered is conveyed over the filter at high speed perpendicular to the direction of filtration.
Ideally, if filters are in the form of pipes or capillaries, the flow moves over them in an axial direction and therefore concomitantly avoids formation of a fairly thick filter cake over the entire internal surface of the membrane.
Suitable membrane filters in pipe and capillary form, which permit the desired dynamic microfiltration, have been available only for a short time. They make it possible to dispense with the emergency solution extended over various stages. The cloudy substance to be filtered can be conveyed to the membrane filter directly without preliminary separation by centrifugation. Since the pipe and capillary membranes are self-supporting, they exhibit adequate stability and high durability. Polypropylene membranes for the most part also display excellent chemical stability and therefore a lack of sensitivity to chemicals used for cleaning.
From U.S. Pat. No. 3,974,068 is known an ultrafiltration process for liquids containing microorganisms, macromolecules or other small solid particles as occur in a fermentation process, e.g., vinegar, wine or cider. In the known process, separation occurs at membranes having pore diameters of 10.sup.-2 to 10.sup.-4 mm and the liquid to be filtered is conveyed parallel to the filter surface. Velocity is allegedly limited in such a manner that a laminar flow is maintained. By way of example, a maximum velocity of 6.76 m/sec is indicated. A filtering aid is added to the liquid to be filtered. This consists preferably of mechanical wood pulp particles. For cooling, a cooler is attached to the membrane filter. The effectiveness of the filtration process is described using vinegar as an example and the filtration results achieved are relatively low.
In the performance of crossflow-microfiltration with wine, comparison of various vintages has shown considerable differences with regard to filtration results and frequency of cleaning the filters. Hence, only qualitative differences in wine vintages could be considered as the cause.
As a natural product, every wine has an individual composition which is also reflected in its filterability. Different filtration processes are therefore used. Red wines are known to be more difficult to filter than white wines. Such differences have been observed not only with wine obtained from grapes but also with fruit wines, although the latter do not otherwise differ from grape wines.
Colloidal components exert a substantial effect on filterability. As was reported in papers by K. Wucherpfennig et al (Deutsche Labensmittel-Rundschau 80(2):38-44, 1984 and Lebensmitteltechnik 5:246-253, 1983), the various colloids behave very differently, with the .beta.-glucans in particular exerting a very negative effect on filterability.
The effects produced by various measures may be as different as the wines themselves and the components affecting filterability.
Since .beta.-glucans are readily soluble in water, coating of the membrane filters with glucan can be controlled by simple rinsing with water. But this causes interruption of the filtration process.
In the case of wines that contain little if any .beta.-glucan, there may be other reasons for poor filter performance.